1. Field of the Invention
The invention relates to a method of producing polarization inversion parts by means of an electric field poling process.
2. Related Art Statement
A second harmonic generation device based on a quasi-phase-matching system has a ferroelectric single crystal, such as lithium niobate or lithium tantalite single crystal, with periodic polarization inversion structure formed therein. The device can generate light of selectable wavelengths in a range from ultraviolet to infrared ray. Such device can be applied in various fields including an optical disk memory, medical, opto-chemical and various kinds of optical measurement uses.
It is necessary to form deep polarization inversion structure in a ferroelectric single crystal, for attaining a high conversion efficiency in a second harmonic wave generation device. According to the method described in Japanese Patent publication No. H11-72809A, the surface of a substrate of a ferroelectric single crystal is inclined with respect to the polarization axis of the crystal by 3° and a comb electrode and rod-shaped electrode are formed on the surface of the substrate. Several low resistance portions are formed between the tip end of each electrode piece of the comb electrode and the rod-shaped electrode. Direct current voltage is then applied on the comb and rod-shaped electrodes, so that polarization inversion parts are formed corresponding with the electrode pieces of the comb electrode as well as the low resistance portions, as shown in FIG. 28 of H11-72809A. Specifically, it is described that the length of the low resistance portion may preferably be 10 to 30 μm and may be 20 μm, for example.